The present invention relates to an improved ball screw device for use in a semiconductor chip mounter, a transfer device in a wood working machine and the like, which require a long stroke operation and high Speed operation, and also to an improved table drive device including the ball screw device.
As a feeding device for a table or the like, which is required the above-mentioned long stroke and high speed operations, it is conceived that a hydraulic cylinder device can be used. However, since the hydraulic cylinder device requires a hydraulic unit, the structure cannot be made compact, and the feeding accuracy thereof is not good.
In view of the above problems found in the hydraulic cylinder device, generally, a ball screw device is used. The ball screw device is advantageous in that it is compact, provides a good feeding accuracy and does not generate a lot of friction or heat. And, in such use, for a high speed operation, especially a ball screw having a large lead is used and, by rotating the screw shaft thereof, a moving table with a nut fixed thereto is moved at a high speed.
However, in the ball screw device, the support span of the screw shaft thereof varies with the movement of the ball nut thereof. That is, the natural frequency of the screw shaft varies according to the positions of the ball nut with respect to the screw shaft and the critical speed of the screw shaft, at which rotation becomes unstable, varies according to the variations of the natural frequency of the screw shaft. In the case of a long stroke, since the support span of the screw shaft is long, the critical speed of the screw shaft is in a low rotation area and, thus, when the screw shaft is rotated at a high speed, the rotation speed of the screw shaft goes beyond the critical speed of the screw shaft. When the screw shaft is rotated in the neighborhood of the critical speed thereof, the screw shaft resonates to produce great vibrations so that the operation of the screw shaft becomes unstable and possibly dangerous. If such a vibrating condition of the screw shaft continues, then the ball screw device may be damaged.
In order to avoid such vibration of the screw shaft, there is proposed a table drive device in which a screw shaft is fixed and a ball nut is rotated to thereby move a moving table.
However, even in a ball screw device of a type that a ball nut is rotated, since the support span of a screw shaft remains long, the natural frequency of the screw shaft itself remains low. Therefore, when the frequencies of minute vibrations to be applied to the screw shaft by the rotation of the ball nut are in the critical speed area thereof, although there vibratory rotations due to eccentric mass are not generated as in the screw shaft rotation type, a resonant phenomenon occurs so that large vibrations are produced.
Thus, even if the vibration of the ball nut to be given to the screw shaft is small, the vibration due to resonant phenomenon causes the screw shaft to be vibrated greatly. In view of this, a table drive device using a ball screw device is generally used at rotational speeds lower than the critical speed of the screw shaft. And, even if a large lead ball screw device is used, the speed of the table drive device is naturally restricted and thus the feeding speed thereof is restricted.
And, in order to realize a high speed, if the screw lead of the large lead ball screw device is made too large, then the drive force of a motor must be made large and thus the inertia of the moving table is increased, which makes it difficult to secure the stop position accuracy of the table.
In view of the above, Japanese Patent Unexamined Publication No. Hei 6-11009 proposes a ball screw device in which a long inner shaft is held as a vibration controlling mass member within a screw shaft including a hollow hole in a coaxial manner, the inner shaft is arranged such that it collides with the inner surface if the hollow hole when the screw shaft vibrates, and such collision restricts the vibration of the screw shaft at the resonating point thereof, thereby being able to realize the operation of the screw shaft at a speed exceeding the critical speed thereof. In this publication, there is disclosed an embodiment in which, when the screw shaft and inner shaft contact each other due to the working precision of the hollow hole of the screw shaft, flexure of the inner shaft, or the like one point of the inner shaft around the central portion thereof is fixed by an annular bush.
However, in the ball screw device proposed in the above-mentioned Japanese Unexamined Patent Publication No. Hei 6-11009, although the long vibration controlling mass member held coaxially by the screw shaft is so arranged as to collide with the inside surface of the hollow screw shaft while it is vibrating, it cannot be always expected that the screw shaft and vibration controlling mass member move relative to each other when the screw shaft is excited at a low frequency. Thus, to control the vibration of the screw shaft at a low frequency, the screw shaft and vibration controlling mass member must be made such that they can carry out their relative movements efficiently. However, in Japanese Patent Unexamined Publication No. Hei 6-11009, no concrete means for realizing the above-mentioned relative movements is taken into consideration and thus it cannot be said that a sufficient damping effect can be expected.